Various processes are known for the recovery of tungstem values from tungsten-containing ores. It is conventional in the prior art to recover from a beneficiated ore a relatively pure tungsten compound, which may be used in the manufacture of other compounds or from which may be extracted metallic tungsten. For example, ammonium paratungstate is used as the source of tungsten in the manufacture of filaments. The tungsten ores contain other chemical materials which may be carried through the recovery process and ultimately contaminate the product. For most applications, it is particularly important to remove the impurities during the recovery process.
The recovery of tungsten values from tungstencontaining ores is shown, for example, in U.S. Pat. No. 1,388,857-Giles et al. This patent does describe the necessity for grinding the initial tungsten ore to a mesh size of about 100 and also describes the alkali leaching of the ore with heat and pressure, followed by calcium treatment for the removal of silica and phosphorus impurities. Calcium tungstate is then recovered from solution, and is acidified and reduced with carbon to recover metallic tungsten.
Amine and organic solvent mixtures are employed to recover ammonium paratungstate according to U.S. Pat. No. 3,052,516-Drobnick et al. The importance of removal of silica as it might apply to the treatment of tungstate solution in the preparation of a purified tungsten product is also shown in U.S. Pat. No. 3,206,303-Goren. However, we have found that operating at a pH substantially different from that shown in Goren, in combination with the other improved features of this invention, provides substantially improved results.
Silica removal is also shown in Burewll U.S. Pat. No. 3,256,057, but a fluorine containing material must be employed for this removal. Obviously, there are inherent difficulties in handling fluorine containing materials, thus making processing according to the patent less practical than would be desired. In addition, Burwell U.S. Pat. No. 3,256,058 uses the fluorine containing materials in addition to a fusion step, but does show the removal of silica impurities following removal of molybdenum impurities, by changing the pH level of the solution to about 8.
While the process as described here, and in the remainder of the specification, is related to an ore concentrate, it will be apparent that an untreated ore could be employed. Preferably, the ore is beneficiated to increase the tungsten content prior to treatment.